We propose to evaluate the response of normal young women to an increase in energy intake under controlled conditions. Measurements will include body composition by K-40 counting, RMR, and thyroid hormone levels; the response of these female subjects will be compared to that of men whom we have studied under similar conditions. The questions to be addressed pertain to the energy cost of the weight gain, the metabolic response, and the relative proportions of lean and fat which constitute this gain. A study of this type does not appear to have been done before in women; the results should be germane to the problem of human obesity, which is more prevalent among women in our society.